The mobile networks according to the invention are Public Land Mobile Networks (PLMNs) that are designed, for example, according to the Global System for Mobile Communications (GSM) standard or according to technologies that inherited from this standard and, in particular, third generation (3G) and Long Term Evolution (LTE) standards.
These mobile networks offer various telephony services including full compatibility with the public switched telephone network and SMS (short messages services). Generally, they offer the capability for mobile terminals to access web sites, download data from the web, beneficiate MMS (Multimedia Message Services) and e-mail services.
A key concept in the architecture of PLMNs is the cell. Indeed, the PLMNs are composed of contiguous geographical areas called cells. For each cell of a PLMN, there is a base station transceiver. This base station is called BTS according to the GSM standard. It is called NodeB or eNodeB according to the 3G or LTE standards, respectively. The base stations communicate with all mobile terminals within a cell. The cell is finally the access point to the network for mobile terminals within its coverage.
Practically, a base station continuously requests identification and authentication from terminals that attempt to use its resource. These are terminals that belong to the same PLMN as the base or not. If the base station recognises a mobile terminal that belongs to the operator's network, the mobile terminal is then allowed to register with the network, using the base station as an access point, and to use the network's communication services as allowed by agreement with the user.
The mobile equipment generally contains a module, which is named, for example, subscriber identification module (SIM). This module is generally a smart card which is inserted in the mobile equipment. The identification data comprise a unique number, which is the international mobile subscriber identifier (IMSI). It also comprises a unique authentication key (Ki), which is associated with the IMSI. The IMSI is made up of three numbers: a country number, a number identifying the PLMN in its country and a number identifying the subscriber by the operator. The Ki is a 128-bit value.
For example, according to the GSM standard, for initial registration of a mobile terminal, the network obtains the IMSI from the SIM of the mobile terminal, and passes this to the mobile operator requesting access and authentication. The operator network searches its database for the incoming IMSI and its associated Ki. It then generates a random number and signs it with Ki associated with the IMSI, computing another number known as the signed response 1 (SRES_1). The operator network then sends the random number to the mobile terminal, which passes it to the SIM. The SIM signs it with its own Ki, producing a signed response 2 (SRES_2), which it gives to the mobile terminal along with an encryption key Kc. The mobile terminal passes SRES_2 on to the operator network. The operator network then compares the computed response SRES_1 with the computed response SRES_2 that the mobile terminal returned. If the two numbers match, the SIM is authenticated and the mobile terminal is granted access to the operator's network. Kc is then used to encrypt all further communications between the mobile terminal and the network. Practically, the network may require the mobile terminal to authenticate every time an event is initiated, every so many events, or even after a certain time period has elapsed. Thus, the SIM is to be inserted in the mobile terminal for authentication all along time. Of course, other authentication methods do exist for standards other than the GSM.
A PLMN operates within an area defined as the territory of competency. Its coverage is limited by the borders of a territory, for example national borders. Outside these borders, a user, who wishes to beneficiate services provided by a foreign locally accessible network and who is not a subscriber of said network, can still access communication and other services that are offered by the foreign locally accessible mobile network, if the local network operator concluded a roaming agreement with the user's own operator. According to such a roaming agreement, the user pays for what he has consumed plus what is called ‘roaming fees’ corresponding to costs of delivering this service. This might result in high communication costs for the user.